1. Field of the Invention
The present invention relates to a thermionic emitter of the type a substantially flat emission surface that is subdivided into conductor sections or interconnects and legs which form current supply terminals as well as serving for securing the emitter.
2. Description of the Prior Art
In directly heated thermionic emitters, an unavoidable temperature gradient arises due to heat dissipation through the contacts. At the hottest points material evaporates in an intensified manner. The decrease of the cross-section of the current-carrying path that results leads to the failure of the emitter due to additional heating, melting or fusing.
Attempts have been made to achieve an optimally homogenous temperature distribution by forming the emitter appropriately and varying its conductor cross-section. The overheating at the hottest points is then lower, so that less material is evaporated, and the lifetime is prolonged accordingly. In practice, however, hot spots always remain, at which the emitter develops a fracture, even if after a longer period of time.
It is an object of the present invention to provide a thermionic emitter of the type described above which achieves an optimally long lifetime prior to a time at which it inevitably fails.
This object is achieved in accordance with the invention in a thermionic emitter of the type initially described, wherein the legs each have a portion having a larger longitudinal electrical resistance (xcexa9/mm) than that of the conductor sections of the emission surface, with the result that areas of maximum temperature are shifted from the middle of the emission surface to its edges as the heating current increases.
The invention is based on the recognition that, in the operation of an X-ray tube, for which such thermionic emitters are highly suitable, the tube current fluctuates according to the application and the nature of the subject that is irradiated. By virtue of the heat discharge via the legs and the resulting heat sink, when the heating current is low the heat discharge via the legs prevails, so that the hottest point is situated in the center of the emission surface. As the tube current increases, the inventive reduction of the electrical resistance in the area of the legs achieves an additional heating in this area, which counteracts the heat discharge and leads to a shift of the temperature maximum from the center of the emission surface to its edge. Ideally the shift of the hottest point should spread over nearly all areas of the emission surface depending on the temperature of the emitter and thus on the tube current, so that in normal operation the areas of intensified evaporation migrate over the entire emission surface, and thus the undesirable melting of a stationary area having an elevated temperature can no longer occur. In this case, material does not evaporate only at one point in an intensified manner; rather, the depletion of material is distributed over a larger surface area. The resulting small change in cross-section leads to an extended lifetime of the emitter.
In a preferred embodiment of the invention, the elevated electrical resistance which is required for the purpose of realizing a temperature-dependent position of the hottest point of the emission surface is achieved forming the portions that have an elevated electrical resistance by means of a reduced cross-section compared to the cross-section of the conductor sections. Preferably, the reduced cross-section is realized by a reduced width of the portions. However, in addition to this, or alternatively, it is possible to bring about the reduction in cross-section by reducing the thickness of the portions, for example by etching.
According to another preferred variant of the invention, the portions having an elevated electrical resistance are situated in the respective areas of the junctions between the legs and the emission surface. When heating currents are low, the function of the portions of elevated electrical resistance as a heat source assumes secondary importance to the fact that the legs function as a heat sink due to the fact that heat is discharged via the legs. As a result, the emission surface has the highest temperature in its middle area. As heating current increases, the function of the portions of reduced electrical resistance as heat sources assumes a primary role, so that the hottest point of the emission surface migrates toward the exterior in the direction of the edge of the emission surface, particularly into the areas of the junctions of the emission surface and the legs.